Mini-Drill Control

18V 8A LM317 LM317 BC547 04. C5 1N4001 B1 10A * see text C1 = C2 Nom 2 47009 35V ELEKTOR ELECTRONICS JULY 1989 SUPPLEMENT 5W The circuit described here is intended as a revolution control for small DC motors as fitted, for instance, in small electric drills (such as used in precision engineering and for drilling print- ed-circuit boards, among others). The behaviour of these motors, which are normally permanent magnet types, is comparable to that of indepen- dently powered motors.In theory, the RPM of these motors depends solely on the applied voltage. The motor adjusts its RPM until the counter EMF generated in its coils is equal to the applied voltage. There is, unfortunately, a drop across the internal resistance of the motor and this causes the RPM to drop in relation to the load. In other words, the larger the load, the larger the drop across the internal resistance and the lower the RPM. The present circuit provides a kind of com- MINI-DRILL CONTROL MJ2500 C3 BC557 C4 17p 40...